In machining, it is important to perform machining according to design drawings and according to machining setting. For that purpose, it is required to precisely find out machining positions, machining directions, and machining amounts with respect to workpieces.
For example, in a structure in which a number of component parts are mechanically coupled together by mechanical coupling parts, such as rivets and fasteners, as in an airframe of an aircraft, it is necessary to perform drilling, which allows the mechanical coupling parts to pass through the respective component parts, with precise machining positions, machining directions, and machining amounts.
When a main wing that is one component part of the aircraft, and a skeleton part or the like are mechanically coupled together by a mechanical coupling part or the like, a protrusion may be formed on the surface of the main wing as the mechanical coupling part protrudes from the surface of the main wing, or a recess may be formed in the surface of the main wing as an attachment hole of the mechanical coupling part becomes deep. The protrusion and recess on the surface of the main wing influence the aerodynamic performance of an airplane. Hence, drilling, which allows a mechanical coupling part to pass through the main wing that is a workpiece, is performed with a precise machining position, a precise machining direction, and a precise machining amount so that the protrusion and the recess are minimized. Here, the machining direction is mainly an angle orthogonal to a workpiece, and it is necessary to obtain the normal vector on a surface to be machined.